Question: Solve for $x$ and $y$ using substitution. ${-2x-y = 7}$ ${y = -x-4}$
Answer: Since $y$ has already been solved for, substitute $-x-4$ for $y$ in the first equation. ${-2x - }{(-x-4)}{= 7}$ Simplify and solve for $x$ $-2x+x + 4 = 7$ $-x+4 = 7$ $-x+4{-4} = 7{-4}$ $-x = 3$ $\dfrac{-x}{{-1}} = \dfrac{3}{{-1}}$ ${x = -3}$ Now that you know ${x = -3}$ , plug it back into $\thinspace {y = -x-4}\thinspace$ to find $y$ ${y = -}{(-3)}{ - 4}$ $y = 3 - 4$ $y = -1$ You can also plug ${x = -3}$ into $\thinspace {-2x-y = 7}\thinspace$ and get the same answer for $y$ : ${-2}{(-3)}{ - y = 7}$ ${y = -1}$